In general, in a spread spectrum communication (hereinbelow abbreviated to SSC) and in particular in a direct sequence (hereinbelow abbreviated to DS), interference excluding characteristics for interference waves other than a desired wave are as indicated in FIG. 6. An information signal (DS signal) spread by a transmitter is correlated with a reference signal within a receiver. When these two signals are matched with each other, the band width of desired waves is returned to the initial value before the spreading. Contrarily thereto, the input such as interference waves, etc., which is not matched, is spread further to a band width greater than the input band width. Consequently, in the case where the desired waves and the interference waves are received, the receiver emphasizes the desired waves, which suppresses influences of the interference waves other than them. Further, since a band pass filter BPF, which makes the desired waves pass through, is used, the interference waves are easily separated.
However, in the case where the level of the interference waves exceeds the processing gain of the receiver, it is not possible to maintain the normal receiving capacity thereof.
Consequently it is necessary to remove the interference waves and e.g. an interference wave eliminating system as indicated in FIG. 7 has been proposed, in which reference numerals 1 and 2 are filters; 3 and 4 are detecting circuits; 5 and 6 are integrating circuits; 7 is a comparing circuit; and 8 is a switching circuit.
The received signal is given to the filters 1 and 2 and the outputs thereof are detected by the detecting circuits 3 and 4, respectively. The detected outputs are integrated by the integrating circuits 5 and 6. The integrated outputs are compared with each other in the comparing circuit 7 and one of the outputs of the filters 1 and 2 is selected by the switching circuit, depending on the result of the comparison. By this system the filters 1 and 2 are used for making the upper side frequency band and the lower side frequency band, respectively, pass through. In this way it is possible a filter output including no disturbing waves.
However, according to the prior art system described above, since the spread spectrum signal has a wide band, the system cannot exhibit its ability against a number of interference waves. For example, in the case as indicated in FIG. 8, it is impossible for the system to remove influences of the disturbing waves.
Further, since a detecting circuit and an integrating circuit are used for every band, this is a burden on the circuit construction and it is not possible to obtain any cheap construction having a small size.
Therefore, in order to have a good received signal even at interference by disturbing waves, the inventors of the present invention have proposed in Japanese patent application Hei 1 (1989)-313813 a spread spectrum receiver, in which an input signal is divided into at least 3 frequency band channels and the interference waves are removed from the input signal by comparing the signals in the respective channels with each other in the magnitude and by removing the signals in the channels, where the magnitude is great, and combining the remaining signals.
However, by the system according to this older application, since the frequency bands, where interference waves exist, are removed by a switch, the spread spectrum signal itself is removed also simultaneously with the removal of the interference waves, which causes worsening of processing (correlation, demodulation) characteristics of the succeeding stages.
Furthermore a system has been proposed, by which only the frequency band, where the highest energy is detected, is removed. However, by this system, in the case indicated in FIG. 10, only a part of the spread spectrum signal is removed and therefore the original object is not achieved.